Steam turbine steampath design consists of a series of stages. Each stage consists of a rotating component and a stationary component. Energy is extracted from each of these stages. The design of a stage is commonly referred to as either "reaction" or "impulse." One of the basic differences in the two design approaches shows up in the distribution of pressure drop through the stage. In a "reaction" design, there is a nearly equal pressure drop in the buckets (the rotating turbine components) and diaphragm (stationary component) for each stage. In an "impulse" design, most of the pressure drop appears across the diaphragm only. A major consequence of this difference in pressure distribution is the development of substantial thrust on the rotor in "reaction" type designs. In a typical HP/IP opposed flow reaction configuration, the HP thrust is partly balanced by a similar but oppositely directed thrust from the IP turbine section.
For example, in converting a "reaction" HP design to an "impulse" design, an unbalance in the thrust is created since the IP turbine section thrust is still present. The magnitude of unbalanced thrust forces in the IP turbine section greatly exceeds the capacity of the thrust bearing. By selectively varying the diameters of the HP rotor in regions with different pressures, however, a compensating "step thrust" can be developed. This is a well known practice which has been used by turbine manufacturers for many years.
During conversion of specific turbine designs from the "reaction" type to the "impulse" type, oftentimes one can only take an educated guess as to the magnitude of the thrust in the IP turbine section. As a result, any deviation between the compensating step thrust and the original actual thrust can lead to severe operational problems for a hybrid "reaction/impulse" configuration. Since the compensating step thrust is determined by the geometry of the rotor, modification to adjust the step thrust would require extensive effort, including disassembly and remachining of the components, including the rotor.
This invention takes advantage of the fact that step thrust can be developed in the HP packing which isolates the HP inner and outer casing. In accordance with one exemplary embodiment of the invention, a bypass hole is drilled through the stationary component (for example, the packing casing or turbine inner shell), parallel to the rotor and spanning several packing rings. The latter may be arranged in groups with different diameters--one group sealing the rotor along a first smaller diameter portion, and a second group sealing the rotor along a larger diameter portion, as determined by a radial step in the rotor. A radial hole is drilled in the casing or inner shell, establishing fluid communication between the axial bypass hole and the adjustable pressure region at the rotor step. The bypass hole is plugged at the downstream or lower pressure side, forcing the flow through the radial hole to the step region of the rotor. The axial bypass hole contains a replaceable flow restrictor or orifice, upstream of the radial hole, sized to achieve a specific pressure at the rotor step. The combination of pressure and rotor step size are instrumental in determining the magnitude of the step thrust. It is desirable, however, to alter the pressure to achieve the desired step thrust rather than to modify the rotor--a considerably more complex and costly route. In accordance with one exemplary embodiment, during initial start-up of the turbine modified as described above, the thrust bearing temperatures are monitored at low load levels. Thrust bearing temperature can be used to determine if the deviation between the original rotor thrust and the compensating step thrust is excessive. If the thrust unbalance is excessive, the orifice can then be easily modified (for example, by replacement with a flow restrictor of different orifice size) to achieve the proper operating thrust balance. To facilitate the adjustment, it is desirable to have access to the flow restrictor from outside the outer turbine shell.
Accordingly, in a first exemplary embodiment of the invention, there is provided a turbine construction which includes a rotor having at least one radial step formed therein, and at least one packing ring assembly mounted in a stationary turbine component and axially spanning the radial step, a thrust balancing arrangement for the rotor comprising an axial passage in the stationary component extending from a high pressure side of the stationary component and in fluid communication with an area adjacent the radial step on the rotor.
In another aspect, the invention relates to a method of converting a reaction type turbine to an impulse type turbine, the impulse turbine including a rotor having at least one radial step formed therein, and at least one packing ring assembly mounted in a stationary turbine component and axially spanning the radial step; the method comprising the steps of:
a) forming an axial passage in the stationary component from a high pressure side thereof to a location adjacent the axial step; PA1 b) placing a replaceable flow restrictor in the axial passage; and PA1 c) supplying high pressure medium to the area adjacent the radial step to thereby create a thrust force on the rotor in a direction opposite to an unbalancing thrust load on the rotor.
Other objects and advantages of the subject invention will become apparent from the detailed description which follows.